(a) Field of the Invention
The present invention relates to an image display device, and a display panel and driving method thereof. More specifically, the present invention relates to an organic electroluminescent (hereinafter, referred to as “EL”) display device.
(b) Description of the Related Art
The organic EL display device, which is a display device for electrically exciting a fluorescent organic compound to emit a light, has organic light-emitting cells that are voltage- or current-driven to display an image. These organic light-emitting cells have a structure composed of an anode (indium tin oxide (ITO)) layer, an organic thin film, and a cathode (metal) layer. For a good balance between electrons and holes to enhance luminescent efficiency, the organic thin film has a multi-layer structure that includes an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL). The multi-layer structure of the organic thin film can also include an electron injecting layer (EIL), and a hole injecting layer (HIL).
There are two driving methods for these organic light-emitting cells: a passive matrix driving method, and an active matrix driving method using thin film transistors (TFTs). In the passive matrix driving method, anode and cathode stripes are arranged perpendicularly to each other to selectively drive the lines. On the other hand, in the active matrix driving method, a thin film transistor and a capacitor are coupled to ITO pixel electrodes so as to sustain a voltage by the capacity of the capacitor. According to the form of the signals applied to the capacitor to sustain the voltage, the active matrix driving method can be divided into a voltage programming method and a current programming method.
The voltage programming method is for displaying an image by applying a data voltage representing gradation to the pixel circuit, but may have a problem of non-uniformity due to a deviation of the threshold voltage of the driving transistor and the electron mobility. The current programming method is for displaying an image by applying a data current representing gradation to the pixel circuit, guaranteeing uniformity. But, this method is problematic in securing the time for charging the load of the data lines, since only a slight quantity of current is used in controlling the organic EL element.
A pixel circuit for compensating for the threshold voltage of the driving transistor in the voltage programming method is disclosed in U.S. Pat. No. 6,362,798 issued to Kimura et al.
The pixel circuit disclosed in U.S. Pat. No. 6,362,798 includes, as shown in FIG. 1, four transistors M1 to M4, and an organic EL element (OLED). The driving transistor M1 transfers a current corresponding to a voltage between its gate and source to OLED, and has a capacitor Cst between the gate and source. The transistor M2 is configured to operate as a diode (i.e., its gate and drain are connected together) and has the gate connected to the gate of the transistor M1. A gate of the switching transistor M3 is connected to a current scan line Sn, and a gate of the transistor M4 is connected to a previous scan line Sn−1.
When the threshold voltage of the transistor M1 is equal to that of the transistor M2, it can be compensated due to the transistor M2. But, when the gate voltage of the driving transistor M1 is higher than the data voltage applied through the transistor M3, the transistor M2 is diode-connected (i.e., configured to operate as a diode) in a reverse direction, as a result of which the data voltage cannot be transferred to the gate of the driving transistor M1. To prevent this phenomenon in the prior art, the precharge voltage VP is applied to the gate of the driving transistor M1 and sustained to be less than the lowest data voltage, while a selection signal is applied to the previous scan line Sn−1. In this manner, the gate voltage of the driving transistor M1 reaches the precharge voltage VP when the data voltage is applied, thereby coupling the transistor M2 in the forward direction.
A current flows through the driving transistor M1 due to a voltage corresponding to the difference between the precharge voltage VP and the power voltage VDD, when the precharge voltage VP is transferred to the gate of the driving transistor M1. This current causes the OLED to emit a light, in which case normal black level cannot be displayed to represent black level gradation. Moreover, the current flows to the OLED while the data voltage is transferred to the gate of the driving transistor M1 and charged in the capacitor Cst, thereby increasing power consumption.